1. Field of the Invention
The invention relates to a DC-to-DC converter for charging an energy-storage capacitor. More particularly, this invention relates to a DC-to-DC converter which is operated using coupled inductor current sensing and a predetermined ON time so that optimum performance of a low-voltage DC power source and the converter in an application such as charging an electronic strobe flash unit capacitor is obtained.
2. Description of the Related Art
A basic DC-to-DC converter such as a flyback type flash charger circuit, employs a low-voltage power source, for example, a battery. In such a converter, the "load" is kept constant; whereas, the internal impedance of the battery changes over the life of the battery. This constant "load" causes poor charging performance and decreased battery life. Current thresholds in the primary and secondary circuits of the converter remain fixed over the life of the battery. As a result, mismatching of the converter "load" to the internal impedance of the battery occurs over the life of the battery.
One solution to maximize battery life and improve converter performance is found in U.S. Pat. No. 4,272,806, assigned to Eastman Kodak Company and hereby incorporated by reference. The '806 patent discloses a DC-to-DC converter circuit which dynamically varies the "load" that the converter places on the battery to maximize the life of the battery. This is accomplished by dynamically matching the converter "load" to the internal impedance of the battery over the life of the battery. In other words, minimum and maximum current threshold values in the transformer or coupled inductors are varied over the life of the battery. One disadvantage of the '806 patent device is that it employs voltage sensing on the primary side of the converter circuit and current sensing on the secondary side. This device utilizes complex control circuitry which is susceptible to noise and is therefore less reliable. In addition, the device is costly.
U.S. Pat. No. 3,541,420, granted Nov. 17, 1970, discloses a DC-to-DC converter having a variable duty cycle driver circuit. In this patent, a transistor driver circuit is connected to the primary of a transformer across whose secondary is connected a capacitor to be charged, and the duty cycle of the transistor is controlled by a current sensor connected in the secondary of the transformer. Specifically, the "ON" time of the driver circuit is fixed for supplying a predetermined amount of energy to the magnetic field of the transformer. When the driver is turned off, the magnetic field collapses, inducing current flow in the secondary of the transformer which is effective to transfer energy from the field to the capacitor. Current continues to flow in the secondary until the magnetic field collapses to zero, that is, until secondary current flow terminates. The driver circuit remains "OFF" as long as the current sensor detects the presence of load current in the secondary. As soon as secondary current terminates, a new cycle is initiated.
A disadvantage of the '420 converter is that it does not provide optimal electrical transfer efficiency. Specifically, the '420 converter requires that the secondary current decay to zero before initiating a new energy transfer cycle. In addition, the '420 converter provides no means for handling noise, induced in the primary winding circuit and the secondary winding circuit, created by the switching ON and OFF of the driver transistor.
In U.S. Pat. No. 4,336,583, issued June 22, 1982, a DC-to-DC converter is disclosed. The '583 converter includes a capacitor connected to the secondary of a transformer having a primary connected to a battery through an input control circuit. When triggered, the input control circuit connects the battery to the primary for only a predetermined time interval whereby current flows in the primary establishing a magnetic field that stores energy derived from the battery. When current in the primary is interrupted, the field collapses, inducing current flow in the secondary thereby transferring energy from the field to the capacitor. The control circuit is triggered by a voltage sensor, located in the primary circuit for monitoring the magnetic field of the transformer, that detects the collapse of the magnetic field to zero. The control circuit can also be triggered by a current sensor, located in the secondary circuit, that detects when secondary current terminates.
As with the '420 converter, the '583 converter suffers from the disadvantage that it does not provide optimal electrical transfer efficiency. Specifically, the '583 converter requires that the magnetic field of the transformer, or the secondary current, decay to zero before initiating a new energy transfer cycle. In addition, the '583 converter provides no means for handling noise, induced in the primary winding circuit and the secondary winding circuit, created by the switching ON and OFF of the driver transistor.
It would thus be desirable to provide a DC-to-DC converter that is simple, cost effective, and that provides a high degree of energy transfer efficiency. It would be further desirable to have a DC-to-DC converter which provides improved battery life and charging performance, and, in addition, high noise immunity.